Belt-type speed change devices or variable transmission units are known, for the controlled driving of any appropriate member from a suitable drive means. For example, such a device may be used on a motor vehicle, for driving, from the motor vehicle engine, an auxiliary component of the vehicle such as an alternator, water pump or fan as are usually found in a motor vehicle.
Such a speed change device may comprise two oppositely disposed coaxial side plates, which are arranged coaxially with each other and one of which is fixed and the other of which is movable relative thereto by resilient return means. The side plates each having frustoconically extending portions defining a tapered groove, and the belt sits within said tapered groove so that when the movable side plate of one pulley moves closer towards its associated fixed side plate, the belt is urged radially outwardly relative to that pulley, riding radially outwardly on the inclined side faces of the groove. Conversely, the belt is displaced radially inwardly on the other pulley, being so permitted by the movable side plate of that pulley moving axially away from the fixed side plate, thus increasing the width between the inclined side faces of the groove in that pulley. The movable side plate has a rest position defined by an abutment whose position is so determined as to provide a given transmission ratio between the two pulleys, as when starting up the transmission device.
In speed change devices of this kind, the belt is invariably subject to wear, so that it must be possible to change the belt when required. In practice, and taking account of the constraints and limitations in regard to installation of such a speed change device, such a drive usually being installed at the front of the engine between the engine block and the water-cooling radiator, it can frequently happen that changing the belt of such a device requires removal of the radiator and other associated components, and is particularly troublesome for that reason.
There are also known speed change devices in which it is possible to displace the movable side plate of one of the pulleys for belt changing, but this involves partial disassembly of the pulley. This disassembly process is obviously inconvenient, and it may happen that a person unfamiliar with the pulley construction may proceed completely to disassemble the pulley, although this is not usually necessary.